1. Field of the Invention
The present invention relates to a water-lubricated compressor capable of preventing freezing of water by discharging water from a water cooler disposed in a water circulation channel of the compressor when there is little or no demand for compressed gas at a compressed gas destination, i.e., a place to which compressed gas is to be supplied.
2. Description of the Related Art
The temperature of screw rotors (hereinafter referred to also as “rotors”) of a screw compressor rises as gas sucked from a suction port is compressed within a rotor chamber. Therefore, a cooling mechanism for cooling the screw rotors is essential. Heretofore, as such a cooling mechanism, there has generally been adopted a cooling mechanism wherein cooling liquid is supplied to the rotor chamber with the screw rotors accommodated therein. As a typical example of a screw compressor using such a cooling mechanism, there is known an oil-cooled screw compressor using oil as the aforesaid cooling liquid. The oil supplied to the rotor chamber not only functions to cool a gas compressing section but also functions to seal and lubricate between the screw rotors and also between the screw rotors and an inner wall of the rotor casing.
In case of using oil as the cooling liquid, the oil is separated and recovered from compressed gas by an oil separating/recovering unit disposed in a discharge channel formed in a compressor body, but a portion of the oil is carried as oil mist to a compressed gas destination together with discharged compressed gas. As a result, in a compressed gas destination requiring clean compressed gas, for example in a manufacturing process for manufacturing electronic parts such as semiconductors, precision machines or foods, there has been the problem that the oil in question adheres to mechanical parts or products associated with the manufacturing process and causes contamination.
In an effort to solve this problem, there has been developed an oil-free compressor which can operate in a dry condition not using oil. In this oil-free compressor, however, there has been the problem that the compression efficiency is greatly deteriorated in a low speed rotation range of the compressor, even if there is made an inverter control as an example. In view of these problems, a water-lubricated compressor using water as cooling liquid has been developed. In the water-lubricated compressor, water is used instead of oil and is allowed to fulfill the functions of cooling, sealing and lubricating. Since it is possible to prevent leakage of gas in the compressing process, a highly efficient compressing action is attained in an overall speed range from low to high speed range and there is obtained a discharge volume of about 30% or more in comparison with that in the dry type.
As in the foregoing conventional oil-cooled compressor, the water used in the water-lubricated compressor is separated and recovered by a water separating/recovering unit which corresponds to the oil separating/recovering unit disposed in the discharge channel, and the water thus recovered is again supplied to the compressor through a water circulation channel. However, when there is little or no demand for compressed gas in the compressed gas destination during the winter season, a water systems including the water separating/recovering unit and the water circulation channel may become frozen, giving rise to a trouble such as breakage.
Now, with reference to FIG. 5 attached hereto, a description will be given about a conventional technique to prevent the aforesaid freezing of water systems in the water-lubricated compressor. FIG. 5 is a front see-through diagram of a water jet compressor having a conventional antifreezing device. According to the conventional antifreezing method in a water jet compressor, two openings are formed in a package 20 and a cover 46 for covering the openings is attached to an outer wall of the package, thereby forming a circulation channel 47, the circulation channel 47 having an inlet port 44 for introduction of air present within the package 20 and a release port 45 for releasing heated air into the package 20, further, air circulating means 42, e.g., an air fan, and heating means 43, e.g., an electric heater, are installed within the circulation channel 47 to circulate heated air through the interior of the package (see Japanese Patent Laid-Open Publication No. 2001-263242).
In the above conventional antifreezing technique it is necessary that the air present within the package be circulated by air circulating means such as a fan for example. However, since a large number of constituent parts are accommodated complicatedly within the package, it is difficult to let the air present within the package circulate thoroughly without stagnation. Thus, according to such an antifreezing method, it is difficult to maintain a constant heating temperature for the entire water system and hence there is a fear that a device particularly apt to be damaged by freezing of water, i.e., a water cooler, may be locally frozen and result in breakage.